Microdialysis probe technology provides access to tissue interstitium for either sampling of diffusible tissue constituents or delivery of bioactive substances. For any analyte of interest, however, the relationship between the analyte concentration in the probe perfusate and in the tissue is a complex function of many factors, such as analyte molecular weight, analyte physicochemical properties, tissue properties, probe membrane properties, probe geometry, and perfusion rate. Better understanding of the relationship is needed in order to improve the quantitative usefulness of the technology. Current studies emphasize two factors in particular. One is the effect of trauma resulting from probe insertion into the tissue. The second is the strong influence of clearance processes in the tissue that remove analyte from the extracellular space, such as cellular uptake, chemical conversion and loss to blood through the microvasculature in the vicinity of the probe. Mathematical modeling incorporating these factors is used to describe diffusive and convective solute transport within the probe and in surrounding tissue. A principal outcome of the models is predictive expressions for the probe extraction efficiency. These modeling efforts have expanded the utility of calibration techniques for determining the extraction efficiency in vivo from perfusate concentration measurements. In addition to permitting estimation of analyte concentrations in tissue extracellular fluid, the magnitude of the extraction efficiency provides quantitative information about the tissue. These quantitative analyses require knowledge of physical properties of the probes, such as diffusive and convective permeabilities of probe membranes, that can be determined under well-characterized conditions in vitro. Applications of these quantitative approaches to microdialysis in the brain are being pursued in connection with alcoholism and studies of drugs of abuse. Other applications involve various normal tissues and tumors. Endogenous solutes of interest include neurotransmitters, particularly dopamine. Examples of exogenous substances employed are Zidovudine (AZT), cisplatin and analogs, fluconazole, ethanol, cocaine and opioids. Validation experiments in animals (mice, rats and primates) involve quantitative autoradiography, histology, and chemical assay of tissue surrounding the probe, as well as measurement of probe perfusate concentrations. In agreement with model predictions, we and others have previously shown that the extraction efficiency for dopamine in the brain decreases with reduction in the rate of extracellular clearance of this neurotransmitter. However, a recent study in the rat striatum suggested that the extraction efficiency may be insensitive to increases in dopamine clearance. We have now shown that this is not the case in mouse nucleus accumbens. In mice treated with a long-acting kappa-opioid receptor antagonist, nor-binaltorphimine, the extraction efficiency was higher than in control animals. Model calculations indicated that the treatment increased the apparent rates of dopamine release and uptake approximately six-fold. For the sake of mathematical simplicity, our previous in vivo models neglected the contribution of solute diffusion in the direction parallel to the axis of the probe. This is often a reasonable assumption considering that the length of the membrane in most probes is much greater than the membrane radius so that the contribution from diffusion perpendicular to the probe axis tends to predominate. In addition, the distance over which diffusion perturbs the solute concentration is inversely related to the avidity of solute clearance from the medium surrounding the probe. For in vivo applications because there is almost always some degree of solute clearance from the extracellular space through which the diffusion occurs. However, paradoxically, no useful mathematical solution exists for purely radial diffusion in the absence of clearance processes. This means that both axial and radial, diffusion must be considered for the usual in vitro applications that lack clearance mechanisms, as well as for in vivo applications in which clearance is slow. A unified model applicable to both in vivo and in vitro situations has been developed by employing finite element analysis to incorporate radial and axial diffusion. Convective exchange of solutes between the perfusate and the tissue is usually assumed to be negligible in comparison to diffusive movement. However, a convective contribution may be difficult to avoid. More importantly, convective enhancement may be desirable for augmenting both the rate of solute delivery and the extent of tissue penetration. Using commercially available probes with 100-kilodalton molecular weight cutoff membranes, we have shown that the hydrodynamic pressure generated by normal effluent flow from the probes can drive a considerable fraction of the perfusate across the membrane. The fraction that is ultrafiltered can be altered in a controlled and linear manner by varying the inflow perfusate flow rate. Alternatively, the ultrafiltration fraction can be altered by varying the vertical position of the effluent collection vial to increase or decrease the hydrostatic pressure within the probe. We have found that both inward, as well as outward, transmembrane flow is achievable by adjusting the height of the collection vial. We have developed a mathematical model to describe the influence of hydrodynamic and hydrostatic contributions to transmembrane fluid flow. We have used the fluid transport model to evaluate the hydraulic properties of the probes from the ultrafiltration factor measurements performed with the probes immersed in well-stirred constant temperature solutions. By adding test solutes, such as fluorescein or radiolabelled mannitol, to either the perfusate or the bathing solutions, we have measured the sampling and delivery solute extraction efficiencies in the presence of transmembrane fluid flow. To enable quantitative interpretation of these measurements, we have expanded our previous mathematical model of solute transport in microdialysis to incorporate the effect of convection either from the perfusate to the tissue or in the reverse direction. For analytes exhibiting concentration linearity, the revised solute transport model predicts that properly defined measures of mass- and concentration-based extraction efficiency are symmetric both in vitro and in vivo, i.e., they possess the same value whether the probe is sampling or delivering analyte. Without this symmetry, most probe calibration techniques would be invalid. The experiments with the test solutes have confirmed this prediction for in vitro conditions. A manuscript describing the mathematical modeling and in vitro validation has been submitted and a companion manuscript on application to convection enhanced delivery of therapeutic agents in vivo is in preparation.